The present invention relates to a digital loop carrier system having a plurality of channel units in a telephone transmission system. The present invention in particular provides for a test access system for channel units and other components of the digital loop carrier system.
Pulse code modulated (PCM) multiplex systems are used in the digital loop carrier systems. The primary purpose of a PCM digital multiplex system is the conversion of voice frequency signals to digital signals using standard sampling and encoding techniques in the multiplexing of 23 additional digitized samples into a T1 serial transmission format. Within the format of the T1 serial bit stream are 24 eight byte locations commonly referred to as the time slots into which the encoded data can be placed. In a typical digital loop carrier system a central office terminal is connected to a remote terminal via a digital transmission line. A typical terminal may have two channel banks of channel units where in each channel bank contains four digroups of 24 channel units. Four digital interface units are used to connect the channel units to the digital transmission line. As is well known in the art the channel units and the digital interface units are computer controlled within the terminal, such as by a bank controller unit.
In general a terminal will receive and transmit analog telephone signals on one side and digital signals on an opposite side. In the prior art there has been a need for having the ability to test the channel units and the digital interface units, as well as other components, for proper operation.
In particular there has been no circuitry or devices available for testing different combinations of signal paths through a terminal or through both the connection from a central office terminal to a remote terminal. The present invention overcomes these drawbacks in the prior art.